Photonic Sensors (2011) Vol. 1, No. 3: 268280 DOI: 10.1007/s13320-011-0011-xPhotonic Sensors Review Fiber-Optic Sensor Applications in Civil and Geotechnical Engineering Wolfgang R. HABEL and Katerina KREBBER FederalInstituteforMaterialsResearchandTesting;DivisionVIII.1:MeasurementandTestingTechnology;Sensors, D-12205 Berlin, Germany *Corresponding author: Wolfgang R. HABELE-mail: wolfgang.habel@bam.de Abstract:Differenttypesoffiber-opticsensorsbasedonglassorpolymericfibersareusedto evaluatematerialbehaviorortomonitortheintegrityandlong-termstabilityofload-bearing structurecomponents.Fiber-opticsensorshavebeenestablishedasanewandinnovative measurementtechnologyinverydifferentfields,suchasmaterialscience,civilengineering, light-weightstructures,geotechnicalareasaswellaschemicalandhigh-voltagesubstations.Very often,mechanicalquantitiessuchasdeformation,strainorvibrationarerequested.However, measurement of chemical quantities in materials and structure components, such as pH value in steel reinforced concrete members also provides information about the integrity of concrete structures. A specialfiber-opticchemicalsensorformonitoringthealkalinestate(pHvalue)ofthecementitious matrixinsteel-reinforcedconcretestructureswiththepurposeofearlydetectionof corrosion-initiatingfactorsisdescribed.Thepaperpresentstheuseofseveralfiber-opticsensor technologies in engineering. One example concerns the use of highly resolving concrete-embeddable fiberFabry-Perotacousticemission(AE)sensorsfortheassessmentofthebearingbehaviourof largeconcretepilesinexistingfoundationsorduringandafteritsinstallation.Anotherexample concernsfiberBragggrating(FBG)sensorsattachedtoanchorsteels(micropiles)tomeasurethe straindistributioninloadedsoilanchors.Polymeropticalfibers(POF)canbebecauseoftheir highelasticityandhighultimatestrainwellintegratedintotextilestomonitortheirdeformation behaviour.Suchintelligenttextilesarecapableofmonitoringdisplacementofsoilorslopes, criticalmechanicaldeformationingeotechnicalstructures(dikes,dams,andembankments)aswell as in masonry structures during and after earthquakes. Keywords:Fiber-opticsensor,monitoring,earthquakedamageassessment,steel-reinforcedstructures,pHsensor, geotextiles, distributed sensor

Received: 21 September 2010 / Revised version: 18 November 2010 The Author(s) 2011.This article is published with open access at Springerlink.com 1. Introduction Fiber-opticsensor(FOS)technologyhasgained worldwiderecognitionduetoitsspecific characteristics.Inengineering,itismainlyusedto monitordeformationand/ortheoverallintegrityof structure components with high level of risks or with highsafetyrequirements.Especiallyin steel-reinforcedconcretestructures,suchfiber-optic sensorsthathavethepotentialtoprovideearly onlineinformationaboutdangerofcorrosionare demanded.Fiber-opticsensor-basedmonitoring methodsarehighlywelcomefornon-destructive assessmentofalltypesofgeotechnicaland engineering structures because these methods: 1)cannot be destroyed by lightning strokes, 2)survive in chemically aggressive environments, 3)canbeintegratedintoverytightareasof Wolfgang R. HABEL et al.: Fiber-Optic Sensor Applications in Civil and Geotechnical Engineering 269structuralcomponents,likeanchorrods,ropes, compositecomponents,layeredconcreteelements, and 4)enabletodeliversensorchainsusingone single fiber. Theuseoffiber-opticsensorsystemscanbe divided, in general, into two main steps: a)Installedorintegratedsensorsareactivated duringtestingthestructureintegritytocollectdata abouttheperformanceofstructurecomponents. Suchmeasurementsaremostlycarriedout periodically,andthesensorstatecanthenbe considered as zero-point state. b)In contrast to this procedure, sensors that have todeliverdatafromthemeasuringobjectwith referencetothepreviousmeasurementmustwork reliablyandstablyoveryears.Theymustnotloose the reference value from the beginning of the signal recording.Therearehighdemandsbothfor operationalsafetyofthesensorsystemandfor reliabilityofallcomponents(long-termstabilityof sensorcharacteristicsandrelatedcomponents, accuracy,repeatabilityofrecordeddata, considerationoftemperatureinfluence,).If zero-point drifts occur, they must be separated from themeasurementinformation.Thistaskmightbe challengingbecausetherecalibrationofsensing elements is, in general, not possible after installation respectively after the first test loading. Inthefollowingsections,theuseofdifferent fibersensortypesbasedonglassandpolymeric opticalfibers(POF)inselectedfieldsofciviland geo-engineering is described. 2. Short-gauge length sensor applications 2.1 Assessment of large concrete foundation piles by fiber-optic acoustic emission sensors 2.1.1 Assessment problem Inordertogetinformationabouttheintegrity and ultimate bearing capacity of large concrete piles inexistingornewlyconstructedfoundations, sensorsareusuallyattachedtothereachablepartof theconcretecomponentsuchasthepilehead. Propagatingwavesarethenexcitedbyanimpactat thepilehead.Thesensorsrecordtheacoustic emission(AE)signalsfromtheconcretestructures. Thebearingcapacityandthepileperformancecan thenbeestimatedusingonedimensionaltheoryof wave propagation. Especially for cohesive soil areas, onecanonlymakevagueassumptionsabout pile-soilinteractionaswellasmaterialandsubsoil conditions.Often,thecommonlyused instrumentationatpileheaddoesnotprovide sufficient information. 2.1.2 Sensor development and model pile tests Inordertoimprovetheperformanceanalysisof largeconcretepiles,highlyresolvingfiber-optic acousticemissionsensorsbasedonextrinsic Fabry-Perotinterferometer(EFPI)sensortechnology havebeendevelopedforembedmentinconcrete pilesatseverallevels.FiberFabry-Perotsensor elements enable recording dynamic signals up to the rangeofseveralhundredskilohertz.Thismethod promises to provide moreprecise information about thepileresponseoverthewholepilelength especially using dynamic pile test methods. Fig.1Castingofsensor-equippedmodelpiles(sensorsare fixedatthecageofreinforcement(left);onepilehasgotan imperfection (left pile in the right picture)). Prefabricatedmodelpiles(Fig.1)areequipped withsensorsandtestedinlaboratorytoprovethe appropriatenessofthedevelopedconcrete Photonic Sensors 270 embeddable sensors which will be subjected to high impact energy during test. Aspecialsensorbodyisdevelopedtowhich differentsensingelementsareattached.The fiber-optic acoustic emission sensors are attached to theinnersurfaceofthesensorbodybyusinga specialtool.Forcomparisonpurposes, accelerometersensorsareinstalledinsideofthe sensorbody;resistancestraingauges(RSG)are attached to the outer surface of the body. Thesetwosmallscalemodelpileshavebeen investigated by dynamic low-strain tests, high-strain andstaticloadtests[1].Allsignalresponsesfrom integrated sensors have been recorded and compared withsignalsobtainedfromcommonmethodsof instrumentation. 2.1.3Testresultsfromlow-strainintegrity method Figure2comparesstrainresponsesofEFPI sensorsfromonepilewithoutimperfection(above) andtheotheronewithimperfection(below)during low-strainintegritytest.Fromtheundamagedpile response(Fig.2,above),itcanbeseenthatthe introducedwavetravelsfrompileheadpassingto the embedded sensors at measuring levels ML1, ML2, andML3tothepiletoeandbacktothepilehead. Thesimulatedgeometricdefectof30-cmlengthin thedirectionofpileaxisatpile2ismarkedwith dotted circles as seen in the lower picture of Fig. 2. Thelengthofthedefect,whichcouldbe calculatedfromthemeasuredtraveltimedifference frompilehead(sensorisinstalledatthepilehead) tothesecondarypeaksatpile2,is31.7cm.By applyingthesameprocedure,howeverusing embeddedsensors,adefectlengthof29.6cm followsfromML1and30.3 cmfollowsfromML3. Thesemeasuredresultsfromembeddedsensorsare definitelybetterconsistentwiththerealdefect length of 30 cm. It must be noted that ML2 does not showsignificantsecondarypeakscausedby superpositionofreflectingwavepartsfromvery closelylocateddefectandtheappliedimpact wavelength.Thetestscarriedoutconfirmthe applicationconceptforFabry-Perotsensorstothe inner surface of the sensor body. 1.00.50.00.51.01.00.50.00.51.01.00.50.00.51.01.0 1.5 2.02.53.0Time (ms)ML1ML2ML3Normalized strain EFPI sensor (-) Pile headPile toe Reflection at pile toe Time (ms)WaveupWave down1.6 1.8 2.0 2.22.42.52.8ML3ML2ML1Pile head Pile toe Pile head Pile head1.00.50.00.51.01.00.50.00.51.01.00.50.00.51.0Normalized strain EFPI sensor (-) Fig. 2StrainresponsesofEFPIsensorsovertimefromone pilewithoutimperfection(above)andtheotheronewith imperfection(below)duringlow-strainintegrity testing;impact wasgeneratedbysteelballwithadiameterof40mm(falling height: 60 cm). 2.1.4 Conclusions from load test in laboratory Strainresultsfromallappliedstraingauges (RSGandEFPI)forhigh-strainpiletestareshown in Fig. 3. Wolfgang R. HABEL et al.: Fiber-Optic Sensor Applications in Civil and Geotechnical Engineering 271EFPI and RSG at 2nd ML EFPI and RSG at 3rd ML EFPI and RSG at 1st ML Common RSG(externally at pile head)High-strain testing time (ms) 123456789 104020020406080100120140160180200Strain (m/m) SG_ML2EFPI_ML1 EFPI_ML3 SG_ML1 SG_ML3 SG0 EFPI_ML2 Fig. 3 Strains measured during high-strain test. ThereductioninEFPIstrainintensityfromone measuringleveltothenextonecausedbyfriction forces can clearly be seen. This reduction in strain is also caused by radiation damping losses at pile skin andtoe.Ontheotherhand,comparisonofinternal strainfromembeddedFabry-Perotsensors (EFPI_ML1,2,3)withresistancestraingauges (SG_ML1,2,3)atthesamelocationateach measuring level reveals that both measuring systems vary in a range of about 5%. Althoughtheconcreteembeddablestrainsensor isdeveloped foracousticemissionsignalrecording, static load tests are carried out as well at both model piles.Loadsareappliedinthreecycles.The maximum load is 110 kN which leads to a permanent pile head settlement of 50 mm in dimension after the thirdloadingcycle.Comparisonofbothsensor systemsrevealsgoodagreement;themeasurement resultsachievedfromEFPIsensorsandresistance strainsensorsdifferintherangeof5%to10%. Comparingtheshapesofstraincurves,itcanbe concludedthatthepileconcretetransfersstrain properlytothesensorbody.Detaileddescriptionof the laboratory tests is given in [1]. The main outcome of the measurement results is that EFPI sensors embedded as described above can beusedforbothdynamicpiletestunderlow-strain aswellashigh-strainloadingconditionsandstatic loadtest.Itispossibletocalculatethebearing capacityofpilesinthesamewayasmadewith standardmethodswhichuseadditionallyembedded accelerometers.Inaddition,calculationscanbe madeonthebasisofmeasurementsatdifferent locationswhichprovidemorepreciseinformation aboutthepilesbearingbehaviour.Therefore,the usuallyrequirednumberofassumptionsfor calculationofbearingcapacitycanstronglybe reduced. 2.1.5 Field tests with adapted sensor bodies Fieldtestshavebeencarriedouttoshowthe abilityofthissensorbodytosurviveveryheavy dynamicimpactenergyontorealprecasteddriven piles. Thedesignofthesensorbodywasmatchedto theconcretemixture(aggregatesize)anddynamic loadingconditionsoftherealconcretepile dimensions.Eightsensorswerelocatedin5 measuringlevelsalonga 19-m-lengthconcretepile. Thelocationsweredefineddependingonthesoil stratification.Figure4showsthecageof reinforcementwiththefixedsensorsbefore concreting.Figure5(left)showsasensor-equipped 19-m-lengthpileduringerection;therightpicture showsthepilesalreadydrivenintosoil.Itcanbe statedthatallsensorssurvivedthehighimpact energyduringthedrivingprocess.SincetwoEFPI sensors and three RSG were attached to each sensor body,atotalnumberof40 straingaugeshadtobe controlled. Fig.4Sensorsfixedatthecageofreinforcementfora 19-m-length concrete pile (above right: sensor body). Photonic Sensors 272 Fig.5Precastedsensor-equippedpilesbeforeandafter driving process. Aspecialmeasuringdevicedevelopedatthe Federal Institute for Materials Research and Testing (BAM)Germanyandaconventionalmulti-channel data acquisition system have been used. A sampling rateof100 kHzperchannelcouldberealizedfor dynamic loading conditions. Onedayafterpiledriving,dynamicloadtesting (DLT)wasappliedtobothpiles.AllEFPIsensors survivedthisloadapplication,too.Whenevera changeinstrainappeared,achangeintheEFPI signaloutputoccurred.Figure6showsthegood agreement between RSG and EFPI from DLT. 200 0 200 400 600 800 1000 3133353739414345474951 53 55Time (ms) RSG ML2EFPI ML2Strain (m/m) Fig. 6ComparisonofstrainfromembeddedRSGandEFPI at ML2 for dynamic load testing. Figure 7 shows the result of the same ML 2 from thestaticloadtesting(SLT).Thecomparisonof bothembeddedmeasuringsystemsshowsagaina very good agreement. Generally, EFPI sensors show lowervaluesinstrainthanRSGatthesame location. Itcanbeconcludedthatfiber-opticextrinsic Fabry-Perotinterferometersensorscanbeusedfor low-strain and high-strain dynamic pile tests as well asstaticloadtests.Fieldtestsshowthatthe developedconcrete-embeddablefiber-opticsensor providesdataforallofthreedifferenttestmethods withonlyonemeasurementsystem.Especially during pile integrity tests (PIT), Fabry-Perot sensors recordverysmalldeformationinsidetheconcrete structure,andbyusinganumberofembedded sensorsdistributedalongthepilelengthatdefined locations,PITallowsthedeterminationofmaterial propertiesbycalculatingthewavevelocityof differentpilesectionsseparately.Thefieldtests showalsogoodagreementwiththeresultsfrom both measurement systems. Details of field tests are describedin[2,3].Thesensorsystemisbeing transferred into a product for commercial use. 200204060801001201401601802002200 1000 2000 3000 4000 5000600070008000 9000 10 000Time (s) RSG_ML2EFPI_ML2Strain (m/m) Fig. 7ComparisonofstrainfromembeddedRSGandEFPI at ML2 for static load testing. 2.2 Evaluation of bearing behaviour of large steel anchors and micro piles 2.2.1 Evaluation problem The classical way to evaluate the bonding of the fixedanchorlengthindifficultsoilareasand/orto carryoutsuitabilitytestsistopullthefixedanchor outandmeasuretheresultingforcesattheanchor head.Thistestmethoddeliversintegralinformation whethertheintroducedanchorforceswillbe transferredintothesoilarea,ornot.However,it doesnotprovideanyinformationhowistheskin frictiondistributionalongthesteelanchorand finallytowhichamountisthelengthoftheanchor involvedintheloadbearing.Inordertoget information on the force distribution of the anchors, fiberBragggratingsensorarrayswereattachedto the surface of steel bars (micro piles with a diameter Wolfgang R. HABEL et al.: Fiber-Optic Sensor Applications in Civil and Geotechnical Engineering 273of80mm)toevaluatetheloadtransferbehaviour. Thismethodwasused,first,toprobemicropiles (typeGEWI63.5 mmZ-32.1-9/1/)thatweretobe used for fixing of foundation plates of a sluice in the German river Weser against uplift. Already in the early 90s of 20th century, optical fiberswereintegratedintoextremelyheavysteel strand anchors (4,500 kN tensile force each) installed tostabilizealargegravitydaminGermany.These sensorsprovidedalong-termmonitoringofthe integrityofthefixedanchorlength.The measurementmethodbasedonopticaltimedomain reflectometry (OTDR) principle is described in [4]. 2.2.2Measurementconceptandsensor installation Inordertofindthemosteconomicmethodto obtainthenecessaryinformation,ameasuring conceptwasdevelopedandimplementedin interdisciplinarycooperationbetweentheFederal InstituteforMaterialsResearchandTesting(BAM) Berlin,NeubauamtfrdenAusbaudes MittellandkanalsinHannover,GermanFederal WaterwaysEngineeringandResearchInstitute (BAW) and the Consulting Office Dietz Geotechnik ConsultHilden/Germany.Thequasi-distributed fiber-opticstrainsensorarrayswereinstalledinthe completelengthofGEWIsteelbar(maximum length:18m)tomeasurestraindistributioninthe loadedanchors/micropiles.Thegaugelengthfor eachsensorinthechainwas200 mm;thedistance betweenthesensorswasbetween750mmand 1500mm.Eachbarwasequippedwithtwosensor arrays for redundancy reasons. Figure8showsfiberBragggrating(FBG) installationintheanchorfactory,andFig.9(left) shows the splice box where sensor fiber and leading cableareconnected.Importanceisreliablefixing andprotectionofthesensorsaswellassafecable egressattheanchorhead(Fig. 9,right)becausethe sensor-equippedanchorshavetobetransportedto the building site over several hundreds kilometers. Fig. 8 Application of FBG sensor to the steel bar. Fig. 9Splicebox(left)andcableprotectionatthepilehead (right). 2.2.3 Tests and results After the anchors were installed (Fig. 10) and the groutwascured,pull-outtestswerecarriedoutin December2006.Thetensileforceduringanchor tests was stepwise increased up to 1,580 kN, and the strain distribution along the fixed anchor length was measured.Fromthemeasuredstraininthefiber Bragggratingsensors,thestraindistributionalong anchorwascalculated.Figure11showstheresult foroneanchor.Abovesolidline,thestrain distribution along the steel rod which is not fixed in soil(freeanchorlength)isplotted;belowtheline, thestraindistributionalongthefixedanchorlength representstheloadtransferintothesoil(load bearing). Fig. 10 Introduction of the micro pile into the bore hole. Photonic Sensors 274 Free length of the anchor Fixed anchor length Force distribution for 300 kN/m2 skin friction Height (m) (above sea level) 15105051015250400600800 110013001580 Fig.11LoadtransferofanchorA1measuredwithapplied strainsensors(diagramsource:GermanFederalWaterways Engineering and Research Institute, BAW). 3. Long-gauge length sensor applications 3.1Distributedsilica-basedfibersensorsfor monitoring of geo-structures with large dimensions Monitoring of extended structures such as dikes, dams, tracks, and highways in mining or critical soil areasrequiressensortechnologieswithgauge lengthsintherangeofseveralhundredsmetersor evenseveralkilometers.Sensingsystemsusing stimulatedBrillouinscattering(SBS)allowthe designoffullydistributedfiber-opticsensorsthat monitorstrainandtemperaturealongopticalfibers overalengthofmorethan10 km.Thismakes them highlysuitablefortheapplicationin geo-engineeringorinextendedstructureslike bridges. BecauseofextremefloodsintwolargeGerman rivers (Oder in 1997 and Elbe in 2002), the national RiskManagementofExtremeFloodEvents (RIMAX)researchprogrammewasstartedbythe German Federal Ministry of Education and Research todevelopintelligentmonitoringsystemswhich wereabletodetectincipienteffectsoffailureof hydraulicengineeringstructures.Theideawasto integratefiber-opticsensorsintogeosynthetics, which were to be used for the stabilization of dikes, damsorsimilargeotechnicalstructures. Geosyntheticsarecommonlyusedindikes,where theyactasfilter,drainageorreinforcement. Equipped with fiber-optic sensors, they form a smart sensingstructurewhichcanbeembeddedinto thesoilalongthelandsidedikefoot,asoutlinedin Fig. 12. Strain in optical fiber due to deformation of geosyntheticsOptical signals Fig. 12 River dike with sensor-based geosynthetics. Incaseofacriticaldeformationofdikebody causedbyerosion,slopefailure,waveovertopping orpiping,thesoildisplacementistransferredto strain experienced by optical fiber. The strain can be detected by a distributed fiber-optic strain sensor. Currently,adistributedcost-effective field-applicablesensingsystemisbeingdeveloped whichisbasedontheBrillouinfrequency-domain analysis(BOFDA)techniqueandfulfilthe requirementsondikemonitoring,havingaspatial resolution of 5 m or better. Thefeasibilityofthisnovelcombinationof coatedopticalfibersandgeosyntheticshasbeen proven in several field tests. Figure 13 shows such a geotextilewithintegratedsensorfibersduringthe installationinagravitydaminSolina/Poland. Measurement system, test results, and application as wellasthetheoreticalbackgroundofthis measurement method adapted to the requirements of geotechnique are described in detail in [5, 6]. Wolfgang R. HABEL et al.: Fiber-Optic Sensor Applications in Civil and Geotechnical Engineering 275Distributedstrainmeasurementusingstimulated Brillouinscatteringinsilicaopticalfibersisquite wellestablished,however,itslimitswhenstrong deformationofthestructureledtostrainofmore than 1%. Fig.13Sensor-basedgeotextilematbeinginstalledduring constructionofagravitydaminSolina/Poland(textile:Saxon Textile Research Institute (STFI) e.V. Chemnitz). 3.2Distributedpolymerfiber-opticsensorfor monitoringofverylargedeformationin geo-structures Slopes,railwayembankmentsorsoilareas whichshowendangeringstabilitybehaviourlike slipping,creepingordepressionhaveusuallyless extended dimensions than dikes, and deformation to be detected can, however, require monitoring systems withthecapabilityofmeasuringstrain(>1%).In suchcases,polymericopticalfiber(POF)hassome advantages. They are able to measure strain of up to 40% and more without significant distortion of light guidingproperties;theyarerobust,alloweasier handling,andhaveanacceptableprice.Currently, the usable length of POF sensor fiber is about 100 m or slightly more. Inordertogetsmartgeotextilesforapplication inareaswithlimitedextensions,polymericoptical fibershavebeenintegratedintononwoven geotextilesusingawarp-knitting technique[7].The knittedfabricscaneitherbeusedasdrainageoras narrowtapewhichposesacarrierforthesensor fiber.Figure14showstwosamplesofsmart geotextiles.Oneimportantpointduringintegration concernsthetransferofgeo-structuresdeformation to be monitored into the polymer optical sensor fiber. Amethodtointegratepolymericfiber-opticsensors without loosing their sensitivity was developed. Fig.14Geotextileswithintegratedpolymericfiber-optic sensors (photo and textile: STFI e. V. Chemnitz). Deformationofthegeotextilecanbeevaluated bymeasuringthePOFelongationusingtheoptical time-domainreflectometryanalysis.Thistechnique iswellknownandmainlyusedtolocateerrorsin fiber-opticcommunicationcables.However,itis alsousedas sensingmethodsuchasmonitoringthe bondingbehaviourofheavyrockanchorsina gravitydaminGermany(referringto[4]).In principle, the time interval from launching the pulse intothefiberuntilthereturnofthebackscattered light(pulseresponsetime)ismeasured.Itdepends linearlyonthedistanceofthescatteringlocation initiated by a local or integral deformation. By using OTDRtechnique,itisknownthatthelevelofthe backscatteredlightincreasesatlocationswhere strainisappliedtoapolymeropticalfiber.This effectisexploitedforthissensingtechnology whereassomespecialquestionsconcerning backscatteringbehaviourduetomechanical influencesarebeinginvestigated.Basically,itmust bepossibletodifferentiatebackscatteredlight causedbystrainfromsuchlightandundesired Photonic Sensors 276 effects such as bending and plastic deformation. For thispurpose,aspecialtestfacilityhasbeen developed and used to introduce strain into the fiber. This test facility allows applying almost any desired strain distribution along the fiber by spooling it onto a coil under defined stress. Figure15showstheincreaseofscatteredlight forappliedstrainupto16%.Therelativelyweak nonlinearity in this strain range becomes stronger for strainvaluesupto40%.Above40%,the polyethylene coating was partially broken which led tosignificantattenuation.Testresultsalsorevealed that the level of scattered light due to constant strain decreased with time. It is being investigated whether thisdecreaseconvergestoastablevalue.Some otherexperimentalresultsandanoutlook concerningperfluorinatedPOFforsensorlengthof more than 100 m are described in [8]. 02468 101214 160 0.5 1.0 1.5 2.0 2.5 3.0 Strain (%) Relative integrated change of scattering Measurement 1 Measurement 2 Measurement 3 Measurement 4 Fig.15Strainmeasurements(4testseries)atdifferent locations along the POF [9]. A very important step in the research programme withPOFistheimprovementofspatialresolution. Some recent results are presented by Lenke et al. in [10].ThestandardPOF-OTDRsensorhasbeen investigated and some new characteristics have been found. It can be shown that the increase of scattering duetotheappliedstrainactuallyresultsfromthe perturbationsofopticalindexinthefibercorethat aresmallerthan350nmindimension.The decreasingeffectofthescatteringlevelhasbeen investigated.Thesuitabilityofusinggradedindex polymericopticalfibersforstrainmeasurementof slowly varying strain is shown and a new procedure toreversethespatialblurthatresultsfrompulse broadening due to modal dispersion is proposed. POFsensorshavealsobeenintegratedin geotextilewhichwerepreparedforconstruction sitesinPolandandGermany(Fig. 13).Itcouldbe demonstratedthatthesensorstructureitselfaswell as the integration procedure into geotextile tape was appropriateforuseon-site.Thesensingfibers endured the construction work of the dam. Inordertoinvestigateandunderstandtheforce transfer mechanisms from the geotextiles to the fiber, straintestswithopticalfibersintegratedintextiles inthelaboratoryoftheresearchpartnerSTFIhave been carried out. Among others, a so-called geogrid sampleequippedwithseveralintegratedPOFwas clampedontwoends,andthenalateral displacementwasforcedwithahydrauliccylinder (Fig.16)[6].Themeasuredresultsshoweda successful integration into the textile and good strain transferfromthetextiletothesensorfibers.For strainvaluesexceeding4%,lossduetomicrobend inthefiberarisingfromdefectsinthetextilewas observed. These defects can be avoided in future by improving textile integration process. Fig. 16LaboratorytestinSTFIe.V.inChemnitz/Germany: forceddeformationofasensor-equippedgeogridand correspondingdifferentialOTDRsignalfordifferentstrain values. In order to prove the sensor function under field conditionsandrevealpossiblenegativelong-term effectsinthesoil,severalsensor-equippedtextiles Wolfgang R. HABEL et al.: Fiber-Optic Sensor Applications in Civil and Geotechnical Engineering 277wereinstalledinarailwayembankmentnear Chemnitz/Germany(Fig.17).Allsensorssurvived theinstallationprocessinvolvingheavymachinery withoutanydamage.Theirlong-termbehaviouris being tested. A measurable strain in the fibers due to non-uniformsettlingisnotexpectedbecauseof heavy pre-compaction of the foundation. Fig.17Installationofsensor-equippedgeotextilesina railway embankment in Saxony/Germany [11]. 3.3Polymericfiber-opticsensorsforassessment ofmasonrystructuresdamagedbynatural disasters Technical textiles are also used for retrofitting of masonrystructures.Thisobjectivearisesespecially fromtheneedofearthquakeprotectionofhistoric buildings.Inordertocombinethestabilizing functionoftextileswiththesignalizationofsevere deformation,afiber-opticsensorsystemfor embedmentisbeingdeveloped.Thissmarttextile enablesstructuralhealthonlinemonitoringof retrofitted masonry structures. The motivation is not only to strengthen the masonry body and enhance its ductilitybyapplyingatextile,butalsotomonitor anddetecttheconditionofthestructuresdamaged orendangeredbye.g.earthquakes.Typicalfaults thatwillbedetectedandmonitoredinthis applicationareverticalcracks.Inthiscase,POF sensorswiththeirveryhighbreakdownstrainand abilitytoresolvethelocationofacrackarevery promising.IthasbeenfoundoutthatPOFsensors notonlyenabledistributedstrainandstrainprofile measurement, but also are appropriate to detect very shortstrainedsectionsinthescaleofafew millimeters with a spatial resolution of up to 10 cm. Firstlaboratorytestshowedthatitwaspossibleto detectlengthchangesof1mmatsmallgauge lengths[9].Thiscapabilityprovestobepromising fortheapplicationofPOFsensorsinarchitectural textiles for crack detection in masonry structures. Small-scalesamplesequippedwithvarious technicaltextileswithintegratedPOFsensorswere fabricatedincooperationwiththetextilepartner STFI Chemnitz/Germany and tested in laboratory of theInstituteofReinforcedConcreteStructuresand BuildingMaterials(IfMB)attheUniversityof Karlsruhe/Germany.Byusingaresinmatrixand commonmortar,thesensortextileswereappliedto one side of two-stone test samples. Figure 18 shows thetestfacilityandthesample;detailsofthetests and related results are described in [9]. Fig. 18 Testsetupwith two-stonesamplewithsensor-textile on the bottom side (inclusion). 4.pHsensorforearlydetectionof potentialdangerofcorrosionin concrete structures Steel-reinforcedconcretestructuressuchas sewerpipes,coolingtowersorrockanchorsare oftenexposedtoawidevarietyofdamaging influences.Asidefrommechanicalstress,corrosion ofsteelisoneofthemostrelevantdamaging processesinsteel-reinforcedconcrete.Itpresentsa safetyrisktopeopleandenvironmentbecause Photonic Sensors 278 failurecanoccurwithoutpriorindication.Besides moistureandchlorideionsconcentration,pHvalue isachemicalparameterofmajorimportancefor healthmonitoringofsteel-reinforcedand pre-stressedconcretestructures.Thelifetimeof steel-reinforced concrete structures depends strongly ontheirpHstateasembeddedsteelsinconcrete structuresareonly passiveatpH valueshigherthan 9.Forthisreason,long-termmonitoringofpH valuesintherangeof9to13witharesolutionof about0.5 pHunitsisrelevantforearlydetectionof potential corrosion condition. Commerciallyavailablestructure-integrated sensorsforearlydetectionofsteelcorrosionin concrete structures do not always sufficiently match thein-siturequirements.Fiber-opticbasedsensors are a promising technology for corrosion monitoring becausetheyofferalargenumberofattractive featuressuchassmallsize,flexibility,geometric versatility,resistanceincorrosiveandhazardous environments,nosignalinterferenceduetopresent moisture,in-situandnon-destructivemeasurement, andimmunityagainstlightningstrokes.Inorderto drawbenefitfromtheseadvantages,a concrete-embeddablelong-termstablefiber-optic pHsensorhasbeendeveloped.Themostchal-lengingrequirementsconcernitslong-termstability under strong alkaline conditions within the pH range between13and9overaperiodofatleast25 years. Thesensorhastobeintegratedinharsh environmentsandinaccessibleplaces.Itmustbe guaranteedthatthesensingelementhasanintimate contactwiththeconcretematrixtopickuppH change.Andfinally,thepriceofpHsensorshasto beaslowaspossibletoenablethefabricationof multiple-sensor structures. Newfiber-opticpHsensorconsistsofa pH-sensitivelayermadeofapHindicator immobilizedinasolidsubstratewhichchangesthe colourdependingonpHvalues.Asaresultof intenseinvestigations,theabsorptionmethodis preferred because only this method provides reliable measurementresults.Inordertoovercome instabilityproblemsresultingfromdecreaseofthe indicatorconcentrationduetophotodegradationor leachingout,driftsofthelightsourceintensityor bending of optical fibers, a ratiometric method based ontheuseofratiobetweentheintensitiesattwo different wavelengths (e.g. at the intensity maximum pointsorattheisosbesticpoint)isapplied.Sucha ratioofintensitiesisnotalteredbyexternalfactors. Themeasurementprincipleandsomemoredetails are described in [12]. Figure 19 shows the design of the sensor head. pH sensitive membrane Teflon holderSteel tubingpH 7 pH 13 Fig. 19 pH sensor head design (the head is pluggable and can bereplaced,andthecolourofthesensitivemembranevaries depending on pH values). Thediameteroftheheadis8 mm;thesensitive layerisprotectedbutsufficientlysensitiveforthe contactwiththeconcretematrix.Thesensitive membranemustnotexceedadefinitethickness becausetheiondiffusionishindered.Inorderto ensureareliablystablethicknessofthesensitive membrane,aspecialpowdercompactingtoolfor manufactureofpH-sensitivemembranesis developed. The measurement resolution of the sensor for pH valuesbetween9and12isintherangeof0.1to 0.6pHunitsdependingonthepHvalues.The highestresolutioncanbeachievedinthemiddleof themeasurementrange(between9.7and11).One particularconditionbynowisthatthepHsensitive membranemustnotdryout.Thisrequirementis mostlyfulfilledinhydraulicengineeringand geotechnique.Inordertopreventdryingoutbefore integration into concrete structures, the pH sensitive membrane is protected by a small watertight topcoat (Fig. 20). Wolfgang R. HABEL et al.: Fiber-Optic Sensor Applications in Civil and Geotechnical Engineering 279 Fig. 20 Water-filled topcoat to prevent dehydration of the pH sensitive membrane (photo: STUMP GmbH). Suitability of this sensor was successfully tested insteelanchorsinstalledintheharbourofRostock (North Germany) in summer 2010. Figure 21 shows twooftotally10pHsensorprototypesthatwere fixedonprefabricatedanchorbodies(twoineach anchor)beforeintroducingthemintotheborehole. Thetopcoat(Fig.20)wasremovedshortlybefore introducingtheanchorintotheboreholeand grouting. This procedure ensured that the membrane maintaineditshydrophilicproperties.Thesensors providedusefulinformationaboutthepHvaluesof the grout. Fig.21PrefabricatedanchorswithfixedpHsensors (arrows). Thelong-termfunctionoftheconcrete embeddablepHsensorcouldbeprovedbyaseries ofinvestigationscarriedoutoverthelast6 years.It couldbeobservedthatthemostseriousattacks concerningstabilityofpH-sensitivemembrane occurred in the first few months. The sensor concept has been optimized for commercial use and is being installed in cooling towers of a power plant. 5. Conclusions Thispaperdescribesdifferenttypesof fiber-opticsensorsusedfornon-dailymonitoring tasksincivilandgeo-engineering.FiberBragg gratingsensorarraysareusedtomonitoranchor deformation in difficult soil areas; fiber Fabry-Perot interferometersensorsareusedasAEsensors embeddedinlargeconcretepilestocharacterizeits integrityandbearingcapacity.Tiny concrete-embeddablepHsensorscanbeinstalledin steel-reinforcedconcretestructurestomonitorthe corrosion-free operation. ByusingBrillouinfrequency-domainanalysis andOTDRtechniqueforfiber-opticsensors,there arenewpossibilitiesfordistributedfibersensors especiallywhentheyareintegratedintechnical textiles.Strainmeasurementsingeosyntheticsand technicaltextilesenablemonitoringofdikes,dams, andunstableorendangeredslopes.Byusing polymer optical fibers with the capability of strain in therangeof1%to40%,monitoringofslopes, railwayembankmentsorsoilareaswithless extendeddimensionswhichshowendangering stability behaviour is possible. Finally, measurement ofcrackopeninginconcreteormasonrystructures helpstoassessstructuresdamagedbynatural disasters. Fibersensorusageon-siterequiresspecial expertiseandexperienceespeciallywhenlong-term measurementshavetobedone.Measurement systemshavetoworkstablyunderverydifferent environmentalconditions,suchastemperature variations,moistureinfluences,chemicalattacksto componentsofthesensorsystem,chemical interactionsbetweenthesensingelementsor specificsensormaterialsandthemeasurandor environment. Packaging and ingress/egress areas for fiber-opticsensorsmustbeveryrobust.However, themostrelevantissueforlong-termmonitoring sensorsystemsconcernstheapplicationofthe sensingelementand/orsensorfiberitself. Application,gluing,crimpingorfixingmustresist cyclingthermalandmechanicalloads.Inorderto considerallquality-relatedaspects,theonlywayis toaimatvalidatedsensorsystemsandvalidated Photonic Sensors 280 applicationmethods.Byusingthismethodology, usersgettheconfirmationthatthesensororthe measurement system works as reliably as demanded. Open AccessThis article is distributed under the terms oftheCreativeCommonsAttributionLicensewhich permitsanyuse,distribution,andreproductioninany medium,providedtheoriginalauthor(s)andsourceare credited. References [1]M.Schallert,J.Stahlmann,andW.R.Habel, StrukturintegrierterfaseroptischerMikrodehnungs-aufnehmerfrdieBewertungvonBetonpfhlenmit dynamischenPfahlprfmethoden,(Structure- integratedfiber-opticmicrostrainsensorfor evalutationofconcretepilesbymeansofdynamic piletestingmethods),inMesseninderGeotechnik 2010,ScientificseriespublishedbyTechnical UniversityofBraunschweig,Inst.forFoundation EngineeringandSoilMechanics,Braunschweig,pp. 265286, 2010. [2]M. Schallert, W. R. Habel, I. Gbel, J. Stahlmann, and P.Wardinghus,Dynamic,andstatictestingof concretefoundationpileswithstructure-integrated fiberopticsensors,inProc.ofthe8thInternational ConferenceonApplicationofstresswavetheoryto piles,Lisbon,Portugal,September,pp.625635, 2008. [3]M.Schallert,W.R.Habel,andD.Hofmann, StrukturanalysevonBetonpfhlendurcheingebettete faseroptischeSensoren(AnalysisofConcrete FoundationPilesUsingStructure-Integrated Fiber-OpticSensorsintm-TechnischesMessen,vol. 75. Mnchen: Oldenbourg-Verlag, 2008, pp. 485500. [4]W.R.HabelandT.Gutmann,Embedded Quasi-distributedFiberOpticSensorsforLong-term Monitoringof4,500kNRockAnchorsintheEder Gravity Dam in Germany, in Proc. of SHMII-2 Conference,Shenzhen,China,BALKEMATaylor& Francis, vol. 1, pp. 289297, 2006. [5]N. Nther, A. Wosniok, K. Krebber, and E. Thiele, A Distributedfiberopticsensorsystemfordike monitoringusingBrillouinopticalfrequencydomain analysis,inProc.Conf.onSmartStructuresand Materials,SPIE,SanDiego,USA,2008,vol.6933, pp. 69330T-169330T-9. [6]A.Wosniok,N.Nther,K.Krebber,andE.Thiele, Distributed monitoring of mechanical deformation in riverdikes,inProc.oftheConf.EurosensorsXXII Dresden,Dsseldorf:VDI-Verlag,2008,pp. 14571460. [7]P.Lenke,S.Liehr,K.Krebber,F.Weigand,and E.Thiele,Distributedstrainmeasurementwith polymeropticalfiberintegratedintechnicaltextiles usingtheopticaltimedomainreflectometry technique,InProc.ofthe16thInternationalPOF Conference, Turin, Italy, Sept., pp. 2124, 2007. [8]S.Liehr,M.Wendt,andK.Krebber,Distributed strainmeasurementinperfluorinatedpolymeroptical fibers using opticalfrequencydomainreflectometry, Meas.Sci.Technol.,vol.21,no.9,pp. 094023-1094023-6, 2010. [9]S. Liehr, P. Lenke, M. Wendt, K. Krebber, M. Seeger, E.Thiele,H.Metschies,B.Gebreselassie,and J.C.Muenich,PolymerOpticalFiberSensorfor DistributedStrainMeasurementandApplicationin StructuralHealthMonitoring,IEEESensorsJ.,vol. 9, no. 11, pp.13301338, 2009. [10]P.Lenke,S.Liehr,andK.Krebber,Improvements of the distributed strain sensor based on optical time domainreflectometrymeasurementinpolymer optical fibers, presented at Proc. 17th Intern. Conf. on Plastic Optical Fibers, Santa Clara, USA, 2008. [11]K.Krebber,P.Lenke,S.Liehr,etal.,Technology andApplicationofSmartTechnicalTextilesBased onPOF,presentedatProc.17thIntern.Conf.on Plastic Optical Fibers, Santa Clara, USA, 2008. [12]N.DantanandW.R.Habel,Monitoringof corrosionprotectionaconcrete-embeddablepH optode,inBFTInternational-ConcretePlant+ PrecastTechnology(Betonwerk+Fertigteiltechnik) Bau Verlag - Springer Science + Business Media, vol. 72, pp. 4855, 2006.